The invention relates generally to the field of particle size measurement during milling and other chemical processes, and more specifically, to an on-line measurement method during the milling process.
Many prior art techniques are used for particle sizing, or determining the mean particle diameter or the particle size distribution. These techniques range from sorting techniques (sieving, Coulter counter), to interactance techniques (ultrasound, optical). Of the interactance techniques, optical techniques are most numerous, and typically rely on light scattering from the particles. This scatter can be time-dependent or wavelength-dependent. Many of these techniques require lasers at a single wavelength, or require light that has been filtered to provide monochromatic light. Some involve forward scatter, and others look specifically at the scatter over certain time domains for using Brownian motion of the particles to determine their sizes.
In the field of near infrared spectroscopy, diffuse reflectance spectroscopy was the first spectroscopy of major practical value (Williams, Phil and Karl Norris, xe2x80x9cNear-Infrared Technology in the Agricultural and Food Industriesxe2x80x9d, American Association of Cereal Chemists, Inc., St. Paul, Minn., 1987). This technique was applied to biological samples, especially wheat, and especially ground wheat. It was observed that the wheat powder or flour gave spectra that were dependent on the particle size of the wheat or flour. In most cases, this dependence on particle size was viewed as a problem to be overcome because the dependence on particle size made it difficult to calibrate the samples for changes in composition, such as the moisture or protein content of the wheat or flour. Many multivariate statistical techniques and algorithms have been developed and designed to reduce, compensate, or eliminate the effects of particle size on the spectra.
In some cases, the particle size dependence of the spectra has been found to be a useful effect. In pharmaceutical grinding of materials, workers have attempted to use this particle size dependence. However, there is little evidence of the use of, or any reference to, the particle size dependence of NIR diffuse reflectance spectra in liquids, slurries, emulsions, and dispersions. U.S. Pat. No. 5,343,044, Lars-Ove Sjaunja, et al., describes the measurement of aqueous fat-containing samples, such as milk, using IR (not NIR) wavelengths. The main objective is to measure the concentration of a component, but the degree of homogenization is also measured, which is an indirect measure of the mean particle size. The materials show lower absorbance with smaller particle size.
Consequently, a need exists for a method of measuring particle size in liquids, slurries, emulsions and dispersions in which there is no correlation of lower absorbance with smaller particle size.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in an on-line method for detecting size of particles in a liquid during a process in which there is not a correlation between low absorbance with small particle size, the method comprising the steps of (a) measuring a plurality of reference particle sizes each associated with a particular near infrared diffuse reflectance spectrum for creating a reference infrared diffuse reflectance spectrum having high absorbance with small particle size; (b) measuring on-line a near infrared diffuse reflectance spectrum of the liquid during the process; and (c) determining the size of the particles of the on-line liquid based on correlating the on-line-measured particle size with the reference near infrared diffuse reflectance spectrum.
The present invention has the advantages of on-line measurement. This allows it to be real-time, and the result can be responded to, either by the operator of the mill, or by using the value to automatically control the mill. For example, the result can be used determine the end point of milling and to shut off the process. Additionally, the measurement is made repeatably, improving precision. Also, the results can be watched with time, showing the operation of the mill as a function of time. The curves generated from this can be used to compare operation of the mill to previous operation, and to predict the future operation of the mill on similar product.